Polygenes
Multi-factorial traits with discontinuous distribution of phenotypes (result of many factors, genes and environment but phenotype is either present or not e.g blood group either A, B, AB or O)
e.g spina bifida
usually a threshold between a combination of genetic predisposition and accumulating environmental factors which once reached the trait shows
can have different threshold for males and females so more common in the sex with the lower threshold
Quantitative traits with continuous distribution of phenotypes e.g height, volume of milk, thickness of wool etc
Polygenic inheritance
many gene loci contributing to the phenotype of the quantitative trait
each locus has two alleles eg., A1 & A2
no dominant phenotype
alleles are inherited in a similar way to any other trait
allele A1 adds to trait contributor
allele A2 does not add to trait non contributor
the effect of the alleles is cumulative
eg., If A1 adds 1cm to height then A1A1 = 2cms
looks like incomplete dominance from F1 but F2 shows difference
As the number of genes controlling a phenotypic character increases the differences between phenotypic classes becomes less distinct.
To calculate number of polygenes: (1/4)^n = proportion of F2 that resemble the parents, where n = no. polygenes. NB: Only use one of the groups that resembles the parents
Heritability: A statistical measure of how strongly the phenotype of the offspring will resemble the phenotype of the parents.
To measure heritability in humans we look at twins and if only one twin has a trait, non concordance, if both have the trait, concordance.